1. Field of the Invention
The present invention relates in general to computer networks and, in particular, to a method for obtaining the best connectivity achievable within Virtual Local Area Networks (VLANs) when these VLANs are mapped onto a computer network managed by the Multiple Spanning Tree Protocol (MSTP).
2. Description of Related Art
The present invention addresses devices at layer 2 of the Open Systems Interconnection (OSI) reference model that are controlled by the IEEE Std 802.1s™-2002 Multiple Spanning Tree Protocol (MSTP) or any other equivalent standard such as IEEE Std 802.1Q™-2003. These devices are often referred to as bridges. And, these bridges are usually grouped together to form a computer network that can be segregated into VLANs. For a detailed discussion about VLANs and how a computer network can be segregated into VLANs, reference is made to IEEE Std 802.1s™-2002 and IEEE Std 802.1Q™-2003 the contents of which are incorporated herein. Inside the computer network, the MSTP provides Multiple Spanning Tree Instances (MSTIs) to forward data frames that are assigned to any given VLAN between bridges. In case of a failure affecting a MSTI (e.g. one of the links forwarding data traffic in a MSTI fails), MSTP might reestablish connectivity in that MSTI by activating some links (i.e. having those links starting to forward data traffic). The activation of some of those links might not ensure that the connectivity within a VLAN mapped onto and associated with that MSTI is maintained. This would result in loss of connectivity within that VLAN, which is undesirable when other links could be activated whose activation would maintain the connectivity within that VLAN.
One way this problem is solved today is by a procedure described herein as “pruning MSTIs” It is believed that the “pruning” of MSTIs can be done by eliminating the MSTI Configuration Messages conveyed in a Multiple Spanning Tree Bridge Protocol Data Unit (MST BPDU) that is received by or transmitted from a bridge port. Such MST BPDUs are exchanged by MSTP-based bridges to construct and then maintain MSTIs. When a port stops receiving MSTI Configuration Messages for a specific MSTI, then the port stops forwarding data traffic onto that MSTI, consequently pruning that MSTI. This solution has a drawback in that it prevents the “pruned” MSTIs from spanning an entire computer network as required by the MSTP standard. As such, this solution is in effect a non-standard solution. Accordingly, there is a need for a new solution that can be used to provide the best connectivity achievable within a VLAN while addressing the aforementioned drawback of the traditional “pruning MSTI” solution. This need and other needs are satisfied by the method, bridge and computer network of the present invention.